1. Field of the Invention
The invention relates generally to the field of dentistry and more particularly to endodontic instruments used to enlarge root canals.
2. Description of the Related Art
In the field of endodontics, one of the most important and delicate procedures is that of cleaning or extirpating a root canal to provide a properly dimensioned cavity while essentially maintaining the central axis of the canal. This step is important in order to enable complete filling of the canal without any voids and in a manner which prevents the entrapment of noxious tissue in the canal as the canal is being filled.
In a root canal procedure, the dentist removes inflamed tissue and debris from the canal prior to filling the canal with an inert filling material. In performing this procedure the dentist must gain access to the entire canal, shaping it as necessary. But root canals normally are very small in diameter, and they are usually quite curved. It is therefore very difficult to gain access to the full length of a root canal.
Many tools have been designed to perform the difficult task of cleaning and shaping root canals. Historically, dentists have been required to use a multitude of tools to remove the soft and hard tissues of the root canal. These tools, usually called endodontic files, have been made by three basic processes: In one process, a file is created by twisting a prismatic rod of either square or triangular cross section in order to create a file with a fluted cutting edge. The second process involves grinding helical flutes into a circular or tapered rod to create a file with one or more helical cutting surfaces. The third method involves “hacking” or rapidly striking a circular or tapered rod with a blade at a given angle along the length of the rod, thus creating a plurality of burr-like barbs or cutting edges. Each of these methods produces an instrument having unique attributes, advantages, and disadvantages.
Endodontic files have historically been made from stainless steel, but due to the inherent stiffness of steel, these tools can pose a significant danger of breakage in the curved root canal. More recent designs have attempted to overcome the problems created by the stiffness of steel. Some attempt to alter the geometry of the stainless steel file in order to provide more flexibility. But, this approach has had only limited success, and the stainless steel tools still have a tendency to break.
A series of comparative tests of endodontic instruments made of nickel-titanium alloy (Nitinol™ or NiTi) and stainless steel were conducted and published in an article entitled “An Initial Investigation of the Bending and the Torsional Properties of Nitinol Root Canal Files,” Journal of Endodontics, Volume 14, No. 7 July 1988, pages 346–351. The Nitinol instruments involved in these tests were manufactured in accordance with fabrication procedures and operating parameters conventionally used in the machining of stainless steel endodontic instruments. This process involved grinding a helical flute in a tapered shaft to form helical cutting edges.
The reported tests demonstrated that the NiTi instruments produced by the described machining process exhibited superior flexibility and torsional properties as compared to stainless steel instruments, but the cutting edges of the instruments exhibited heavily deformed metal deposits which, according to the article, rendered the instruments generally unsatisfactory for clinical use.
In general, alloys of nickel (Ni) and titanium (Ti) have a relatively low modulus of elasticity (0.83 GPa) over a wide range, a relatively high yield strength (0.195–690 MPa), and the unique and the unusual property of being “superelastic” over a limited temperature range. Superelasticity refers to the highly exaggerated elasticity, or spring-back, observed in many NiTi and other superelastic alloys over a limited temperature range. Such alloys can deliver over 15 times the elastic motion of a spring steel, i.e., withstand twisting or bending up to 15 times greater without permanent deformation. The particular physical and other properties of Nitinol alloys may be varied over a wide range by adjusting the precise Ni/Ti ratio used. However, the superelastic properties of NiTi also make the material very difficult to machine.
For these reasons, machining of NiTi tools for endodontic use has been an area of significant development efforts in recent years. For example, U.S. Pat. No. 5,464,362 to Heath et. al. describes a method of grinding a rod of a nickel-titanium alloy in order to create a fluted file. However, this process remains relatively expensive and slow.
Accordingly, there is a need for an improved production process which will allow for more economical manufacture of an endodontic tool from nickel titanium alloys and similar super-elastic materials having increased flexibility and versatility.